1. Field of the Invention
The invention relates to an ultrahigh pressure discharge lamp of the short arc type in which the mercury vapor pressure during operation is at least 150 atm. The invention relates especially to an ultrahigh pressure discharge lamp of the short arc type which is used as the back light of a liquid crystal display and for a projector device using a DMD, such as a DLP or the like.
2. Description of Related Art
In a projector device of the projection type, there is a demand for illumination of images onto a rectangular screen in a uniform manner and moreover with adequate color reproduction. Therefore, the light source is a metal halide lamp which is filled with mercury and a metal halide. Furthermore, recently smaller and smaller metal halide lamps, and more and more often point light sources have been produced and lamps with extremely small distances between the electrodes, have been used in practice.
Against this background, instead of metal halide lamps, lamps with an exceptionally high mercury vapor pressure, for example, with 150 atm, have been suggested recently. Here, the increased mercury vapor pressure suppresses broadening of the arc (the arc is contracted) and a clear increase of the light intensity is the goal. Such an ultrahigh pressure discharge lamp is disclosed, for example, in Japanese patent disclosure document JP HEI 2-148561 (U.S. Pat. No. 5,109,181) and in Japanese patent disclosure document JP HEI 6-52830 (U.S. Pat. No. 5,497,049).
In such an ultrahigh pressure discharge lamp, the pressure within the arc tube during operation is extremely high. In the side tube parts which extend from each side of the arc tube portion, it is therefore necessary to place the silica glass of which these side tube parts are formed, the electrodes and the metal foils for power supply sufficiently, and moreover, tightly, directly adjoining one another. If they are not arranged tightly adjoining one another, the added gas escapes or cracks form. In the process of hermetic sealing of the side tube parts, therefore, the silica glass is heated, for example, at a high temperature of 2000° C., and in this state, the silica glass with high thickness is gradually subjected to shrinking. In this way, the adhesive property of the side tube parts is increased.
However, if the silica glass is heated to an unduly high temperature, the defect arises that, after completion of the discharge lamp, the side tube parts are often damaged, even if the adhesive property of the silica glass on the electrodes or the metal foils is increased.
This defect is caused by the following:
After heat treatment, in the stage in which the temperature of the side tube parts is gradually reduced, as a result of the differences between the coefficient of expansion of the material of the electrodes (tungsten), and the coefficient of expansion of the material of the side tube parts (silica glass), there is a relative difference of the amount of expansion. This causes cracks to form in the area in which the two come into contact with one another. These cracks are extremely small. However, during lamp operation, together with the ultrahigh pressure state during operation, they lead to crack growth; this causes damage to the discharge lamp.
In order to eliminate this disadvantage, an arrangement as shown in FIG. 9 is suggested. In the figure, the light emitting part 2 of a discharge lamp 1 is adjoined by the side tube parts 3. The tips of an electrode 6 and an electrode 7 project into the light emitting part 2 and on their respective ends, hereinafter also called the upholding parts of the electrodes, the electrodes are each connected to a metal foil 8. A respective coil component 10 is wound around the areas of the electrodes 6, 7, which are installed in the side tube parts 3. This arrangement reduces the stress which is exerted on the silica glass by the coil components 10 which have been wound around the upholding parts of the electrodes as a result of the thermal expansion of the (upholding parts of the) electrodes. This arrangement is described, for example, in Japanese patent disclosure document HEI 11-176385.
However, in reality, there was the disadvantage that, in the vicinity of the electrodes 6, 7 and the coil components 10, there remain cracks, even when the thermal expansion of the electrodes is accommodated by one such arrangement. These cracks are admittedly very small, but there are often cases in which they lead to damage of the side tube parts 3 when the mercury vapor pressure of the light emitting part 2 is roughly 150 atm. Furthermore, in recent years, there has been a demand for a very high mercury vapor pressure of 200 atm and beyond to 300 atm. At this high mercury vapor pressure during operation, the growth of cracks is accelerated. As a result, there was the disadvantage that noticeable damage to the side tube parts 3 occurs. This means that the cracks grow gradually during lamp operation with a high mercury vapor pressure, even if they were extremely small at the start.
It can be stated that the avoidance of cracks under these conditions is a new technical object which was never present in a mercury lamp with a vapor pressure during operation of roughly 50 atm to 100 atm.